In the design of antennas for communications satellite systems, there are several important design considerations. The desired antenna should provide maximum signal gain, introduce minimum noise into the system and exhibit relatively low side-lobe signal levels. Such receiving antennas typically utilize a prime focus feedhorn to illuminate a parabolic reflector so as to achieve the best compromise among the listed design considerations.
To provide maximum signal gain, uniform illumination across the entire parabolic reflector is desirable but conflicts with the requirement for minimum noise and low side-lobe levels which demand a highly tapered illumination. Tapered illumination refers to illumination of the center of the reflector and utilizing the outer edge of the reflector as a shield from thermal noise radiated from earth.
Theoretically, the minimum noise and maximum gain requirements of antenna design can be met by uniformly illuminating the parabolic reflector with a feedhorn which emits a signal having infinitely steep side boundaries of its signal pattern hereafter "skirts"). Practically, such illumination can only be approached by selecting a parabolic reflector having a focal length to diameter ratio (f/D) matched to the performance of an optimized feedhorn.
To optimize carrier (signal)-to-noise ratio (C/N), consideration must be given to the amplifier to which the feedhorn is coupled. While ten years ago, very high temperature amplifiers (on the order of 600 Kelvin (K.)) were used, commonly 100 K. are now the industry standard with 75 K. units becoming available.
One well-known prior art feedhorn available on the market today maximizes C/N on a 0.375 f/D antenna using a 120 K. amplifier. The feedhorn comprises a circular waveguide having a corrugated plate disposed around the outside of the aperture at one end of the waveguide and including a 1/4 wave transformer at the other end of the waveguide for impedance matching and coupling to the amplifier. See for example, U.S. patent applications Ser. Nos. 271,815 or 322,446, now U.S. Pat. No. 4,414,516, filed by the inventor and assigned to the assignee hereof, and incorporated by reference as if fully set forth herein. Such a feedhorn provides relatively uniform illumination across the parabolic reflector, its characteristic signal over the bandwidth of interest having relatively steep skirts and a substantially flat top by properly selecting the diameter of the circular waveguide for the center frequency of interest, and by properly locating the corrugated plate with respect to the outside of the aperture of the waveguide.
With advances in amplifier technology, the need for further advancement of antenna technology is clear. Broad bandwidth and wide beamwidth for uniform illumination of the parabolic reflector and steep side skirts of the emitted signal pattern is required to meet improved amplifier performance. The ideal signal pattern is flat-topped, having infinitely steep skirts. Furthermore, the pattern should be approximately equal (symmetrical) in the E and H planes which are orthogonal to each other.
E and H plane symmetry is desirable because most communications satellites in use today emit two orthogonal signals which must be received. To achieve E and H plane symmetry the aperture of the feedhorn in the E plane should be smaller than that in the H plane. This configuration arises because the electric field of the H plane is sinusoidally distributed across the diameter of the waveguide and there is no current in the sidewalls of the waveguide. However, the electric field of the E plane causes current to flow in the sidewalls of the waveguide which, upon reaching the aperture, flows down the outside of the waveguide and makes the aperture appear larger. Thus, by reducing the E plane dimension appropriately, the critically equivalent apertures for approximately equal E and H plane beamwidths are produced.
A circular waveguide is used in most present-day feedhorns because it is the most convenient way to receive the two orthogonal signals transmitted by communications satellites. However, obviously it is not possible to reduce only E plane beamwidths by reducing the aperture of a circular waveguide in one dimension without simultaneously affecting the other dimension which affects H plane beamwidth.
It is well understood that signal beamwidth can be controlled by changing aperture size. The smaller the aperture, the wider the pattern for both the E and H plane beamwidths. It is also well understood that beamwidth can be controlled by adding a plate around the aperture of the circular waveguide of the feedhorn, such plates having various configurations, sizes and location behind the aperture. Depending on location, the aperture of the circular waveguide appears to protrude beyond the plane of the plate.
Location of the plate with respect to the aperture primarily affects the E plane beamwidth since it is interacts with the current flowing down the outside of the waveguide. When the current reaches the plate, it is reflected back toward the aperture. If that current is at the proper amplitude and in the proper phase when re-introduced at the aperture, it augments the signal pattern emitted by the feedhorn. An equivalent explanation found in the literature refers to excitation of higher order modes which reinforce the principal TE11 mode in the waveguide.
If the diameter of the aperture of the circular waveguide is reduced by decreasing the diameter of the waveguide along its entire length, severe impedance mismatch is produced. To overcome that impedance mismatch at the center frequency of interest, the circular waveguide must be lengthened substantially. The longer the waveguide, the more unwieldy the feedhorn is to mount, rotate or otherwise conveniently use. According to the present invention, however, H plane signal beamwidth can be controlled by reducing the diameter of the circular waveguide just at the aperture by insertion of a small annular iris. Impedence match of the feedhorn is thus only slightly compromised.
In practice, location of the plate around the aperture affects both the E and H plane signal patterns. The effect is greater for the E plane than for the H plane, which is expected because of the E plane current flowing in the walls of the waveguide.
A feedhorn constructed in accordance with the principles of the present invention comprises a circular waveguide having a corrugated plate disposed around the outside of the aperture of the waveguide wherein the corrugations of the plate are capacitive as to E plane signals. In addition, the feedhorn of the present invention includes a reduced aperture diameter which selectively protrudes beyond the plane of the corrugated plate. The amount of protrusion of the aperture is determined to approximately equalize E and H plane beamwidths and selectively shape the top and skirts of signal pattern around the center frequency of interest. Aperture diameter is reduced primarily to control H-plane beamwidth for uniform illumination across the entire area of the parabolic reflector.